Environmental and conservation considerations have focused new attention on the existing ways and means of disposing of waste heat from large industrial installations such as power plants. A modern steam turbine electric power plant has a thermal efficiency of only about forty percent, meaning that 60 percent of the heat energy put into the system has to be eliminated as waste. In the majority of such plants, the heat is removed by drawing water from a stream or a lake or even a body of salt water, running it through the plant once and then returning the heated water to its source. This type of cooling is becoming less acceptable as questions arise about the adequacy of cooling water supplies and about the biological effects of warming natural bodies of water.
Two well-known alternatives to the "once through" cooling method are in common use. These are the "wet" and "dry" cooling tower system. In the dry system, heat is exchanged from the service water to air. While air is a perfectly good coolant, huge volumes of the gas must be moved and elaborate heat exchange surfaces must be provided in order to cool a large plant effectively. The initial cost of such a system is high.
A more practical alternative is the "wet" cooling tower, where both air and water serve as coolants. In such a tower, the service water being circulated through the plant comes in intimate contact with circulating air and heat is carried away, mostly through the process of evaporation. The rest of the water is collected at the bottom of the tower and returned to the system to perform its function in the steam condenser where the water again is heated. Because the tower cooling is mainly evaporative, part of the service water circulating through the system is lost. The rule of thumb is one percent of evaporation loss for each ten degrees farenheit of cooling.
There are two other sources of water loss; the first is drift, meaning water that is carried away from the tower as very fine droplets suspended in the air leaving the tower. The second source of loss results from the need to constantly bleed off part of the water and dispose of it in order to maintain a substantially fixed level of dissolved solids in the water. Without the bleed off, or "blowdown", as it is called in the art, the evaporative loss of water causes increasingly higher concentrations of dissolved solids in the remaining water. The addition of "make up" water to replace that lost by evaporation and drift does not sufficiently dilute the high concentrations of dissolved solids to prevent the solids from reaching or surpassing a level of saturation and then fouling the tower by forming scale on parts of the system and thus interfering with its operation. It becomes necessary to actually remove from the system some of the water with high dissolved solids concentration and add more make up water to keep the system operational with a level of dissolved solids that can be tolerated which must be below saturation. As a rule, blowdown water amounts to about 0.3 percent of the total system service water for each ten degrees farenheit of cooling achieved by the tower.
The need for the blowdown process creates two significant problems. The first problem is that of disposing of the blowdown water, which, by definition, is polluted with a high concentraton of dissolved solids. The second problem is one of water conservation. Although drift losses require make up, this portion of the total loss is relatively small, usually not exceeding 0.2 percent of the total water circulated in a well designed tower. The sum of the losses, however, including the loss due to blowdown, is significant in a large cooling tower. For example, in a modern 680,000 kilowatt generating plant, the amount of water circulated through the tower units under average summer conditions would be about 345 million gallons per day and the requirement for make up would be about 6.5 million gallons per day, a considerable portion of which could be saved if the blowdown operation were not required.
Therefore, it is the primary object of this invention to eliminate the requirement for liquid waste blowdown in a wet cooling tower operation by providing a method and means for removing the dissolved solids from the service water in a solid crystalline form.
A second object of the invention is to conserve fresh water resources by providing a method and means for effective use of make up water in a heat rejecting system whereby the quality of the make up water is not important; that is, low quality or industrial waste water havng total dissolved solids approaching saturation or having noxious dissolved or suspended solids would be acceptable into the system.
A third and important object of the invention is to utilize heretofore wasted heat energy for a socially valuable function, i.e. the disposal of polluted water.
A further object of the invention is to resolve environmental issues that delay and obstruct the construction of industrial and power plants by providing apparatus which will meet ultimate environmental requirements of no liquid waste discharge and no thermal pollution. This same object may be met by providing additional industrial plant siting alternatives.
A further object of the invention is to provide means for removing dissolved solids from cooling tower water where a substantial part of the cost thereof is chargeable to the waste heat rejection function and the dissolved solids removal cost is minimal.
An additional specific object of the invention is to provide means that can function either as a conventional cooling tower with conventional controls and water chemistry or as both a cooling tower and a remover of dissolved solids from service water.
Another object of the invention is to provide means for economic recovery from water of commodity chemicals.
A further object of the invention is to provide a means of water pollution abatement which allows the employment of many different water treatment chemicals since the waste water is not discharged.
A still further object of the invention is to provide a means of water pollution abatement capable of removing selected dissolved solids by adding chemicals to the waste water to develop insoluable compounds which are precipitated out of solution and then mechanically separated.
Another object of the invention is to provide a means for disposal of liquid waste water produced by water reclamation and purification processes, such as ion exchange, reverse osmosis, electrodialysis, multistage flash and vapor compression.
Although the prior art has practiced many methods for the removal of dissolved solids from large quantities of water such as liquid waste blowdown waters, such methods have not been noteworthy for their success, either economic or substantive. Some of these methods have included a process of reverse osmosis followed by brine disposal, ion exchange followed by brine disposal, mechanical evaporation sometimes followed by evaporation blowdown disposal, and the use of chemical precipitation and solar ponds. In view of the sometimes high cost and general ineffectiveness of these methods per se and in further view of the large quantities of heat associated with cooling tower operation, it is a further object of this invention to beneficially utilize otherwise wasted heat energy in creating a more effective and economical means of removing dissolved solids from large quantities of water, even where that object is paramount and without regard to waste heat rejection, such as, for example, the removal of certain chemical elements or compounds from sea water.